Ti: sapphire crystal fiber, manufacturing method thereof, and wide band light source using the same

ABSTRACT

The present invention relates to a crystal fiber, and more particularly to a Ti: sapphire crystal fiber, a manufacturing method thereof, and a wide band light source with the same. The Ti: sapphire single crystal is grown by means of laser-heated pedestal growth (LHPG) method into a crystal fiber of a predetermined diameter. The crystal fiber is enclosed by a glass capillary and is grown into a single cladding crystal fiber. The wide band light source comprises: a pumping source for providing a pumping light; a single cladding Ti: sapphire crystal fiber for absorbing the pumping light and emitting the wide band light.

FIELD OF THE INVENTION

The present invention is related to a Ti: sapphire crystal fiber,manufacturing method thereof, and wide band light source using the same.

BACKGROUND

In the current application of Ti: sapphire (Ti: Al₂O₃) available on themarket, a crystal block is mainly used as gain medium to be applied in aTi: sapphire laser. There is a significant limit on practicalapplication due to a bulky laser device, resulted from a need for a highwattage pumping source.

Some academic groups have invested the research and development of thestructure of Ti: sapphire integrated optical waveguide, which isfabricated mainly by growing a Ti: sapphire crystal on a plate, and thenetching the grown Ti: sapphire crystal. The manufacturing process isdifficult and complicated, as well as the finished waveguide still farfrom commercialization due to its considerably high transmission loss.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide a crystal fiber,particularly to a Ti: sapphire crystal fiber, manufacturing methodthereof, and wide band light source using the same.

It is another object of the present invention to provide a Ti: sapphirecrystal fiber comprising a Ti: sapphire crystal single crystal core anda glass cladding, constituting an optical waveguide structure havinglower transmission loss compared with a single crystal fiber with nocore.

It is a further object of the present invention to provide a Ti:sapphire crystal fiber having a core with a diameter of less than 50 μm,allowed for enhancing luminous efficiency of a wide band light source.

It is a further object of the present invention to provide amanufacturing method of a Ti: sapphire crystal fiber capable of growinga Ti: sapphire single crystal rod into a crystal fiber having apredetermined diameter through a simple manufacturing process by meansof LHPG method substantially.

It is a further object of the present invention to provide amanufacturing method of a Ti: sapphire crystal fiber capable ofmanufacturing a Ti: sapphire crystal fiber with high quality and smalldiameter through multiple crystal growths by means of LHPG method.

It is a further object of the present invention to provide amanufacturing method of a Ti: sapphire crystal fiber, in which the Ti:sapphire crystal fiber is annealed by laser heating so as to enhanceoutput power.

It is a further object of the present invention to provide amanufacturing method of a Ti: sapphire crystal fiber, in which a singlecladding Ti: sapphire crystal fiber is manufactured by LHPG method, soas to lower transmission loss and enhance output power.

It is a further object of the present invention to provide a wide bandlight source device using a Ti: sapphire crystal fiber, in which asingle cladding Ti: sapphire crystal fiber is mainly used forfabricating a wide band light source.

It is a further object of the present invention to provide a wide bandlight source device using a Ti: sapphire crystal fiber, in which asingle cladding Ti: sapphire crystal fiber is used to greatly reduce thevolume of the device, enhance luminous efficiency, reduce requirementfor light collimation, and increase system stability.

It is a further object of the present invention to provide a wide bandlight source device using a Ti: sapphire crystal fiber, in which apumping light source is selected from a frequency-doubled laser or ablue diode laser having a wavelength of 532 nm.

To achieve the above objects, the present invention to provide a wideband light source device using a Ti, comprising: a core made of Ti:sapphire single crystal; and a cladding covering the outside of saidcore.

The present invention further provides a manufacturing method of a Ti:sapphire crystal fiber, comprising: providing a Ti: sapphire singlecrystal rod; growing said single crystal rod into a crystal fiber havinga predetermined diameter by means of LHPG method; annealing said crystalfiber; providing a glass capillary into which said crystal fiber isplaced; and growing said crystal fiber covered inside said glasscapillary into a single cladding Ti: sapphire crystal fiber by means ofLHPG method.

The present invention further provides a wide band light source deviceusing a Ti: sapphire crystal fiber, comprising: a single cladding Ti:sapphire crystal fiber; and a pumping light source used for providing apumping beam onto one end of said crystal fiber.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the manufacturing by means of LHPG methodaccording to one embodiment of the present invention.

FIG. 2 is a diagram showing the growth of crystal fiber according to oneembodiment of the present invention.

FIG. 3 is a diagram showing the annealing according to one embodiment ofthe present invention.

FIG. 4 is a diagram showing the manufacturing of a single cladding ofthe present invention.

FIG. 5 is a flow chart illustrating the manufacturing method of a singlecladding crystal fiber of the present invention.

FIG. 6 is a diagram showing a wide band light source according to oneembodiment of the present invention.

FIG. 7 is a power diagram of the single cladding crystal fiber of thepresent invention.

FIG. 8 is an output frequency spectrum of the single cladding crystalfiber of the present invention.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, there are shown diagrams illustrating themanufacturing using Laser-Heated Pedestal Growth (LHPG) method and thegrowing of crystal fiber, respectively, according to one preferredembodiment of the present invention. As illustrated in these figures,the Ti-sapphire (Ti: Al₂O₃) crystal fiber is fabricated mainly by LHPGmethod. Substantially, in a laser-heated device 10, a first fixture 181is used to clamp a Ti: sapphire single crystal rod 22, and a secondfixture 183 is used to clamp a matted crystal 24.

After a laser beam 11 generated by CO₂ laser is directed into thelaser-heated device 10, the collimated light beam may be converted intoa ring light beam via a first conic mirror 121 and a second conic mirror123. Subsequently, the ring light beam may be in turn reflected onto aparabolic mirror 16 via a reflective mirror 14, and then focused on theend face of the Ti: sapphire single crystal rod 22.

The end face of the Ti: sapphire single crystal rod 22 may be melted toform a melting regime 221 because it is heated by laser beam. At thismoment, the matted crystal 24 is allowed to contact with the meltingregime 221 and then pulled up slowly, while the Ti: sapphire singlecrystal rod 22 is pushed up more slowly. Thus, a crystal fiber 26 withthe same crystal orientation as that of matted crystal 24 may be grown.Various ratios of diameter reduction may be achieved by the use ofvarious speed ratios between the growing speed of matted crystal 24 andthe speed of pushing single crystal rod 22. For instance, if the speedratio between the growing speed of matted crystal 24 and the speed ofpushing single crystal rod 22 is 16:1, the diameter ratio between thegrown crystal fiber 26 and the single crystal rod 22 is then 1:4.

A general single crystal rod cut from a Ti: sapphire single crystalblock is a square bar of approximate 500 μm×500 μm. The LHPG method maybe performed two, three, or more times for growing, so as to obtain acrystal fiber of better quality and thinner diameter. For instance, thesingle crystal square bar is previously grown into a crystal fiberhaving a diameter in the range between 250 μm and 320 μm, followed by asubsequent growth which results in a reduced diameter in the rangebetween 80 μm and 180 μm, and in turn a further growth which results ina reduced diameter of less than 50 μm.

Referring to FIG. 3, there is shown a diagram illustrating annealingaccording to one embodiment of the present invention. In the Ti:sapphire crystal fiber, fluorescence is generated when Ti³⁺ is pumped.During the process of growth with diameter reduction using the LHPGmethod, however, a part of Ti³⁺ may be oxidized into Ti⁴⁺ withoutfluorescence generated. Moreover, ions of Ti³⁺ and Ti⁴⁺ may be matched,leading to the absorption of a wide wave band centered at the wavelengthof 800 nm, which is just coincided with fluorescent wave band, thusseriously affecting optical efficiency. Therefore, an annealing processfor reducing Ti⁴⁺ into Ti³⁺ is necessary after the Ti: sapphire crystalfiber is grown with diameter reduction to be one having a predetermineddiameter.

In this connection, this annealing process may be performed in afurnace. Namely, the Ti: sapphire crystal fiber is placed into thefurnace with vacuum or filled with hydrogen and inert gas, and thenannealed at high temperature in the range between 1600° C. and 2000° C.

Also, the annealing process of the present invention may be performed bylaser heating. As shown in FIG. 3, a capillary 34 with high softeningpoint may be firstly placed into the Ti: sapphire crystal fiber 26.Either a fused silica capillary with high softening point of 1600° C.,or a quartz capillary with higher softening point may be used. Thecapillary 34 with high softening point containing the Ti: sapphirecrystal fiber 26 therein may be placed into the laser-heated device,with one end of the capillary 34 with high softening point being meltedby laser heating to form a sealing 36, and the other end thereof beingeither filled with the vacuum as indicated by an arrow 38, or refilledwith hydrogen and inert gas. The laser beam 11 is enabled to focus ontothe Ti: sapphire crystal fiber 26, so as to heat this Ti: sapphirecrystal fiber 26 until the maximum temperature that the capillary withhigh softening point can withstand is reached.

A slow motion of the Ti: sapphire crystal fiber 26 together with thecapillary 34 with high softening point is allowed so as to anneal eachpart of the Ti: sapphire crystal fiber 26. The circumstance for thisannealing may be filled with the vacuum or a controlled anaerobiccircumstance. Moreover, an annealed Ti: sapphire crystal fiber 32 isprovided with more Ti³⁺ by which strong wide band fluorescence may begenerated.

Referring to FIG. 4, there is shown a diagram illustrating themanufacturing of a single cladding of the present invention. Theannealed Ti: sapphire crystal fiber 32 is placed into a glass capillary.This glass capillary may be selectively a borosilicate capillary 42 ormade from other types of optical glass with softening point lower than1000° C. The borosilicate capillary 42 containing the annealed Ti:sapphire crystal fiber 32 may be placed into the laser-heated device,with one end of the borosilicate capillary 42 being heated by laser toform a sealing 46, and the other end thereof being filled with thevacuum as indicated by an arrow 48. The laser beam 11 is enabled tofocus onto the annealed Ti: sapphire crystal fiber 32 for heating ituntil a temperature higher than the softening temperature of theborosilicate capillary 42 is reached.

The softened borosilicate capillary 42 may be attached onto the annealedTi: sapphire crystal fiber 32 owing to the pressure difference betweenits inner vacuum and outer pressure, in such a way a single cladding 44may be formed. A slow motion of the annealed Ti: sapphire crystal fiber32 together with the borosilicate capillary 42 may be enabled, so as toheat each part of the annealed Ti: sapphire crystal fiber 32 to becovered by the single cladding.

The refractive index of the core in the Ti: sapphire crystal fiber ofthe single cladding structure is greater than that of the cladding 44.Thus, an optical waveguide structure may be formed.

Referring to FIG. 5, there is shown a flow chart illustrating amanufacturing method of a single cladding crystal fiber of the presentinvention. As illustrated in this figure, the manufacturing method ofthe present invention comprises the steps follows. In step 501, firstly,a Ti: sapphire single crystal rod is provided. The single crystal rod isgrown with diameter reduction several times by means of LHPG method, insuch a way that a Ti: sapphire crystal fiber having a predetermineddiameter, such as 10 μm to 50 μm, for example, may be grown, as shown instep 503.

The annealing process for the Ti: sapphire crystal fiber having apredetermined diameter may be performed selectively by the furnace, andalso by laser heating, as shown in step 505. Subsequently, in step 507,the annealed Ti: sapphire crystal fiber is placed into a glass capillaryhaving an appropriate bore (for instance, 50 to 100 μm for the innerdiameter, and 80 to 170 μm for the outer diameter).

Finally, in step 509, the Ti: sapphire crystal fiber covered inside theglass capillary is grown into a single cladding Ti: sapphire crystalfiber by means of LHPG method. In this connection, a capillary ofoptical glass material having a softening temperature lower than 1000°C., such as borosilicate capillary, for example, may be selected as thisglass capillary.

Referring to FIG. 6, there is shown a diagram illustrating a wide bandlight source device according to one embodiment of the presentinvention. As illustrated in this figure, a wide band light sourcedevice 60 of the present invention comprises a single cladding Ti:sapphire crystal fiber 68 and a pumping light source 62.

In this case, the pumping light source 62 is used for providing apumping beam 621. Preferably, the pumping light source 62 is afrequency-doubled laser having a wavelength of 532 nm, or a blue laserdiode having a wavelength of 446 nm. It is preferable that a diameter ofa core in the single cladding Ti: sapphire crystal fiber 68 is smallerthan 50 μm, as mentioned above.

The wide band light source device 60 further comprises a focusing unit661, provided between the pumping light source 62 and the singlecladding Ti: sapphire crystal fiber 68, used for focusing the pumpingbeam 621 onto the core of the single cladding Ti: sapphire crystal fiber68. Ti³⁺ of the core is capable of absorbing the pumping light and thenemitting wide band fluorescence, which may be turned into self-amplifiedemission generated in the optical waveguide structure of the singlecladding Ti: sapphire crystal fiber 68, and finally radiated from theother end of the crystal fiber 68 as a wide band beam 681.

The wide band light source device 60 further comprises a first filter641 provided at the other end of the crystal fiber 68. The wide badebeam 681 may be utilized on condition that the remaining pumping lightis filtered by the first filter 641.

Moreover, between the pumping light source 62 and the crystal fiber 68,there is further provided with a second filter 643, used for filteringunnecessary residual light at 808 nm and 1064 nm of the output of thefrequency-doubled laser at 532 nm. Between the single cladding Ti:sapphire crystal fiber 68 and the first filter 641, there is alsoadditionally provided with a collimation unit 663, used for collimatingthe wide band beam 681, facilitating the use of the wide band beam 681at the backend.

Referring to FIG. 7, there is shown a power diagram of a single claddingTi: sapphire crystal fiber of the present invention. In the operation ofthe single cladding Ti: sapphire crystal fiber of the present invention,as illustrated in this figure, the output power of 213 μW may beobtained for the wide band beam when the absorption rate of the pumpinglight is 136 mW. In this case, the power conversion efficiency isapproximately 1.5×10⁻³.

Referring to FIG. 8, there is shown a frequency spectrum of the outputof the single cladding Ti: sapphire crystal fiber of the presentinvention. For the single cladding Ti: sapphire crystal fiber of thepresent invention, there is generated fluorescence with a centerwavelength of 759 nm, bandwidth (full width at half maximum or 3-dBbandwidth) of 181 nm, optical spectrum coverage over the wave bandbetween 670 and 850 nm, and coherence length in air of 1.45 μm, asillustrated in this figure. The frequency spectrum is substantiallydistributed near infrared regime, just as the part being least absorbedby body tissues. Thus, it is suitable for the application for opticalcoherence tomography (OCT). Additionally, in optical tomography, thevertical resolution up to 1.45 μm is achieved, while the strengthdistribution of measurement 82 of frequency spectrum shown in FIG. 8 isextremely close to the distribution of Gaussian fit 84. This perfectGaussian waveform may lead to extremely small side lobes of interferencesignal and an extremely little cross-talk of longitudinal image whenthis wide band light source is applied for optical tomography. Thereby,a three-dimensional image with high picture quality may be realized.Indeed, an excellent wide band light source is obtained.

The foregoing description is merely one embodiment of the presentinvention and not considered as restrictive. All equivalent variationsand modifications in shape, structure, feature, and spirit in accordancewith the appended claims may be made without in any way from the scopeof the invention.

The invention claimed is:
 1. A Ti: sapphire crystal fiber, comprising:an annealed core made of Ti: sapphire single crystal defining an outsidefirst circumferential face having a first refractive index; and acladding of a different material having a second refractive index, saidcladding defining an inner second circumferential face covering theoutside first face of said core, said first and second circumferentialfaces defining a laminar refractive interface, wherein said secondrefractive index is different than said first refractive index.
 2. TheTi: sapphire crystal fiber according to claim 1, wherein said core has adiameter of less than 50 μm.
 3. The Ti: sapphire crystal fiber accordingto claim 1, wherein said cladding is made of borosilicate material. 4.The Ti: sapphire crystal fiber according to claim 1, wherein saidcladding is made of optical glass with softening point lower than 1000°C.